Human papillomavirus (HPV) vaccination is a safe and effective primary prevention strategy for cervical cancer and other anogenital cancers, yet vaccination rates are remarkably low. Only 53% of adolescent females have initiated the vaccine (received 1 dose). More importantly, far fewer (35%) have completed the 3-dose series, which is vital for effective cancer prevention. Moreover, there are growing disparities in HPV vaccine completion rates, with Hispanic females significantly less likely to complete the series than non-Hispanic Whites. Low rates of vaccine completion are of crucial concern for populations disproportionally affected by HPV-related disease such as Latinas, whose cervical cancer incidence rates are 60% higher than in non- Hispanic Whites. Although studies have examined predictors of HPV vaccine initiation, there is a fundamental gap in the literature about predictors of series completion. Because receipt of all 3 doses is essential for maximum protection against HPV, identifying factors that influence whether Hispanic adolescents complete the series is critical for informing future intervention efforts. The long-term goal of ths research program is to develop effective strategies for increasing HPV vaccine completion among low-income Hispanic adolescents, thereby reducing morbidity, mortality, and disparities associated with HPV-related disease. The objective of this application is to determine key predictors of HPV vaccine completion, with special emphasis on identifying modifiable psychosocial and cultural factors associated with completion of the series. We propose two specific aims: (1) Determine key predictors of HPV vaccine completion (receipt of all 3 doses) among low-income Latina mothers of adolescents who have received the first dose of HPV vaccine and (2) Provide complementary qualitative information about facilitators and barriers that play an important role in HPV vaccine completion. To achieve Aim 1 we will conduct a yearlong longitudinal study of low-income Latina mothers whose adolescent daughter/son initiates the HPV vaccine. At baseline, both mothers and their daughter/son (aged 9-17) will complete a survey to assess cultural (e.g., acculturation) and psychosocial factors (e.g., perceived barriers to completion, subjective norms for completing the series) from major theories of health behavior (Health Belief Model, Theory of Planned Behavior). These variables will be used to predict our primary outcome variable: HPV vaccine completion status, as verified by the child's medical record. To achieve Aim 2 we will conduct in-depth qualitative interviews with a purposive sample of mothers from the longitudinal study. The goal of these interviews is to provide greater insight into cultural/psychosocial factors associated with completion, to identify factors related to completion not reflected in traditional theories of health behavior, an to investigate factors that may arise after mothers complete the baseline assessment. Outcomes from this project are expected to have an important positive impact by advancing knowledge needed to develop effective strategies for increasing HPV vaccine completion rates among low-income Hispanic adolescents.